But I continue to assert that the reason we use two carabiners in a top rope belay, even when no longer needed for security, is to increase the overall bend radius (effective sheave diameter) and decrease the probability of damaging the rope fibers.

No way! The master point on a TR is a single point of catastrophic failure of the system. The amount of motion at that point and the fact that it is unattended is the real reason it often receives two carabiners. Carabiners connecting belay/rappel devices can be checked and managed by the user. A toprope anchor master point cannot. It could be a misguided attempt to avoid friction for you but everyone else I have ever met who uses two opposed biners does so to prevent any event that could free the rope from the system. An experienced climber can understand when two biners are superflous or supersitious. Beginners should continue to learn rules that will ultimately contribute to an increased level of safety in situations they cannot properly judge. Through experience everyone learns why rules can be bent or broken.

This^^^

I'll just add that the experienced climber will eventually run into the situation in which the two rope-end biners prefer to sit in such a way that their gates are both facing straight out away from the wall. I've most commonly found this at the top of sport climbs with closely spaced anchor bolts.

In such a situation, the experienced climber will recognize that they're not doing anyone any favors by following the default, and will instead orient both gates *away* from the rock.